1. Field of the Invention
The present invention relates to non-linear optical silica thin films, particularly non-linear optical silica thin films having a plurality of regions which differ in polarization orientation and a method of manufacturing such a thin film.
2. Description of the Related Art
It is proposed to provide an optical wavelength conversion element or the like which converts fundamental waves into predetermined harmonics with a function of optical conversion by periodically forming polarization inversion regions in a ferroelectric material having a non-linear optical effect so as to have quasi phase matching to incident waves. As the ferroelectric material, a bulk crystal, such as LiNbO3 (lithium niobate) or LiTaO3 (lithium tantalate), which is placed in advance under the control of single directional polarization orientation, is employed. Further, it has been proposed that polarization inversion regions are periodically formed in such a bulk crystal by selectively applying direct current voltage and irradiating a high energy ray to the bulk crystal.
For example, Japanese Patent Laid-Open Publication No. Hei 2-187735 (JPA2-187735) discloses that, as shown in FIG. 1, on a first main surface of LiNbO3, crystal 1 which is subjected to single directional polarization orientation, a first electrode 2 having a striped shape which is applicable to periodical polarization inversion structure to be obtained is formed, and on a second main surface of the crystal 1, a second electrode 3 covering the entire surface is formed. Periodical polarization inversion regions having a pattern which corresponds to that of the first electrode 2 are formed on a surface of the crystal 1 by applying predetermined direct current voltage between the stripe-shaped electrode 2 and the second electrode 3.
Further, Japanese Patent Laid-Open Publication No. Hei 6-242480 (JPA6-242480) discloses that, as shown in FIGS. 2A to 2C, polarization inversion regions penetrating from the surface to the back surface of a crystal substrate are periodically formed by first providing single directional polarization orientation to LiTaO3 crystal and then selectively irradiating a high energy ray to the crystal.
However, in order to periodically form polarization inversion regions in a bulk crystal as described above, a process of forming the polarization inversion regions is required, as well as the primary process of forming the crystal. Further, there is another problem that an amount of polarization inversion regions which can be formed is restricted. For example, in order to periodically form a polarization inversion structure by forming a stripe-shaped electrode as shown in FIG. 1, an electrode patterning process is required and also a process of applying voltage to the electrode is required. Further, since no electrodes for applying voltage can be formed in the bulk crystal 1, the periodical polarization inversion structure cannot be formed in a direction of thickness of the bulk crystal 1. On the other hand, in a method of forming the polarization inversion structure in the bulk crystal by irradiating a high energy ray as shown in FIGS. 2A to 2C, the electrode patterning process is not required, but a polarization orientation process has to be applied to the bulk crystal by irradiating a high energy ray according to a pattern to be formed. Further, since a high energy ray is irradiated from a surface of the crystal substrate, no periodical polarization inversion structure can be formed in a direction of thickness of the crystal substrate unlike the case of FIG. 1 described above.
Further, non-linear optical materials, such as LiNbO3 and LiTaO3, which are used as crystal substrates of the periodical polarization inversion structure are bulk type crystals. Thus, there are problems that it is difficult to carry out fine processing and have a high degree of functionality, and a process of combining with other function elements, such as a semiconductor element, is required. Further, when LiNbO3 or the like is used as an optical element, an optical loss may arise because there is a great difference in physical properties between the LiNbO3and glass often used as a connecting member (for example, optical fiber glass).
Further, it has been proposed that a silica material which does not differ so much from glass in physical properties, specifically an SiO2xe2x80x94GeO2material, is employed as a non-linear optical material which is possible to be a thin film. However, this non-linear optical material is still under study. Thus, an appropriate and concrete structure of film and its manufacturing method for the purposes of using a non-linear optical silica thin film as an optical conversion element have not yet been proposed.
An object of the present invention is to provide a non-linear optical silica thin film having desired polarization orientation structure in the thin film.
In order to achieve the object mentioned above, the present invention is made and characterized in that the inside of a non-linear optical silica thin film is polarization oriented by forming the film irradiating charged particles at the time of producing the non-linear optical silica thin film.
By forming a non-linear optical silica thin film irradiating charged particles, distribution of charges arises inside the silica thin film being formed, and according to the distribution of charges, polarization orientation in the silica thin film is automatically controlled to be in a desired state. Thus, almost simultaneously with the completion of forming the non-linear optical silica thin film, a polarization orientation process required for obtaining a non-linear optical effect is completed.
Further, the present invention is characterized in that by the repetition of forming a thin film in a state of irradiating charged particles and forming the thin film in a state of irradiating neutral particles or in a state of non-irradiation of particles when the non-linear optical silica thin film is formed, a plurality of regions in different states of polarization orientation are formed in a direction of film thickness of the silica thin film mentioned above.
In the case of a silica thin film area formed by irradiating charged particles and a silica thin film area formed in a neutral state, a high distribution of charges arises in the film. Therefore, a direction of polarization in a material of the thin film between these two regions are oriented according to the direction of distribution of charges. When a non-linear optical silica thin film is formed by the repetition of forming thin film in a state of irradiating charged particles and forming thin film in a neutral state, a plurality of regions in different states of polarization orientation are formed in a direction of film thickness of the silica in such a manner that the polarization orientation is periodically inverted, whereby a periodical polarization orientation structure for quasi phase matching applicable to optical conversion elements can be easily formed in the film.
Further, in the present invention, when a non-linear optical silica thin film is formed, a plurality of regions in different states of polarization orientation may be formed in a direction of film thickness of the silica thin film by the repetition of forming the thin film by irradiating positive particles and forming the thin film by irradiating negative particles.
Further, in the present invention, it is preferable that in the manufacturing method described above, while shifting the irradiation from particles of one polarity to particles of the other polarity, the silica thin film is formed without further carrying out the irradiation process of particles described above or irradiating neutral particles to the silica thin film.
By forming the silica thin film repeating the irradiation of positive and negative particles, more preferably, interposing a process of forming silica thin film in a neutral state between the irradiation of positive and negative particles, higher distribution of charges arises between a positive particle irradiation region and a negative particle irradiation region of the non-linear silica thin film. Thus, polarization of materials of the silica thin film between these regions is automatically oriented according to a direction of the distribution of charges. Consequently, a plurality of regions in which the conditions of polarization orientation periodically differ can easily be formed in a direction of film thickness of the silica thin film by repeating such a process.
Further, according to another aspect of the present invention, in the manufacturing method described above, a plurality of regions in different states of polarization orientation are formed in a direction of film surface of the silica thin film selectively masking the charged particles to irradiate and irradiating the charged particles to the non-linear optical silica thin film.
By selectively masking charged particles to irradiate, the charged particles are selectively irradiated to the non-linear optical silica thin film, which is being formed, in a direction of its film surface, and selective distribution of charges arises in the irradiated regions, whereby polarization in the film is oriented toward a predetermined direction. Therefore, a plurality of regions in different states of polarization orientation can be formed in the film also in a direction of the film surface simultaneously with the completion of forming the silica thin film.
Further, a non-linear optical silica element according to another aspect of the present invention has a non-linear optical silica thin film, and in the non-linear optical silica thin film, a plurality of regions in different states of polarization orientation are formed in a direction of its film thickness.
Since a plurality of regions in different states of polarization orientation are formed in a direction of film thickness of the silica thin film, if an area of the silica thin film formed in a direction of the surface is a desired area when the silica thin film is formed, for example, polarization orientation regions can be periodically formed in a wider area in a direction of the film thickness and it is easily applicable to an optical conversion lens.
Further, the present invention is characterized in that the non-linear optical silica thin film described above is mainly composed of an SiO2xe2x80x94GeO2.
In a non-linear optical silica material whose main component is the SiO2xe2x80x94GeO2 described above, four-coordinate elements Si and Ge bond together via an element O, thereby forming coordination bonds of an Sixe2x80x94Oxe2x80x94Si, an Sixe2x80x94Oxe2x80x94Ge, and a Gexe2x80x94Oxe2x80x94Ge. It is considered that a part which causes the material to manifest non-linear characteristics among these coordination bonds is part of a bonded portion of a Gexe2x80x94O where dangling bonds (unpaired electrons) exist because not all the bonds of four-coordinate element Ge link with the element O and a part of the bonds is left out without being bonded. It is considered that if charged particles are irradiated at the time of forming such a silica thin film material, distribution of charges will arise in the thin film, and all the dangling bonds which exist in the thin film will be oriented toward a predetermined direction according to the distribution of charges arose, whereby polarization orientation will arise in the thin film simultaneously with formation of the silica thin film.